A diamond is expected to be an ultimate semiconductor substrate. This is because a diamond has a lot of excellent characteristics, which are unparalleled in anywhere as a semiconductor material, such as high thermal conductivity, high electron/hole mobility, high dielectric breakdown field strength, low dielectric loss, and a wide bandgap. The bandgap thereof is about 5.5 eV which is a very high value in existing semiconductor materials. Particularly, in recent years, UV light emitting elements using a wide bandgap, field effect transistors having excellent high-frequency characteristics, and the like have been developed.
When it is considered that a diamond is used as a semiconductor, a certain size such as a diameter of several inches is required. This is because when a processing device which is used for micro-machining of a general semiconductor such as Si is applied to a diamond, it is difficult to apply the processing device to a small substrate less than several inches.
Several ideas have been proposed as the method of causing a diamond having a certain size to grow. Among the methods, a diamond single crystal growing method (so-called mosaic growth method; for example, see Patent Literature 1) of arranging plural small diamond single crystal substrates or a manufacturing method (for example, see Patent Literature 2) of using a single-crystal magnesium oxide (MgO) substrate as a base substrate and forming a diamond film on the base substrate by a heteroepitaxial growth method can be used as a strong candidate.
The mosaic growth method is a technique of growing and forming a large diamond single crystal substrate by arranging plural diamond single crystal substrates in a shape of tiles and causing diamond single crystals to newly grow on the diamond single crystal substrates using a homoepitaxial growth method. However, coupling boundaries are formed as areas in which crystal quality deteriorates on the boundaries between the diamond single crystal substrates arranged in the shape of tiles. Accordingly, a coupling boundary is necessarily formed in diamond single crystals obtained using the mosaic growth method.
The reason of formation of the coupling boundary is that diamond single crystals grow randomly in the area of the coupling boundary, coalescence occurs from various directions, and a large amount of potential is generated in the coupling boundary. The coupling boundary is a distinct boundary line which can be observed visually.
Since the part of the coupling boundary cannot be used for growth of a semiconductor device, an area which can be used in practice is limited with respect to the area of the diamond single crystal substrate which is obtained by the mosaic growth method.
To make matters worse, the area of the diamond single crystal substrate which can be used to manufacture a semiconductor device does not match the size of a semiconductor device chip necessarily. Accordingly, in the process of manufacturing a semiconductor device in the diamond single crystal substrate, it is necessary to perform the process to avoid the coupling boundary. As a result, the process of manufacturing a semiconductor device is complicated.
On the other hand, the heteroepitaxial growth method is a technique of causing a diamond film which will be a diamond substrate to epitaxial-grow on a base substrate formed of a material having different physical properties. Since one diamond film epitaxial-grows on one base substrate, there is no concern that a coupling boundary between plural diamond single crystal substrates is formed like the mosaic growth method.
Accordingly, among the two methods of the mosaic growth method and the heteroepitaxial growth method, the heteroepitaxial growth method is particularly prospective in that a substrate area in which a semiconductor device can be manufactured is not easily restricted.
However, a stress is generated in crystals of a diamond substrate formed by growth due to a difference in lattice constant and thermal expansion coefficient between the base substrate and the diamond and thus a warp or a crack is generated in the diamond substrate. Accordingly, it is not easy to obtain a large substrate using the heteroepitaxial growth method.
Therefore, several prior arts relevant to a decrease in stress generated in a diamond formed by the heteroepitaxial growth method have been reported (for example, see Patent Literature 3).    Patent Literature 1: Japanese Patent No. 3387154    Patent Literature 2: Japanese Patent No. 5066651    Patent Literature 3: Japanese Unexamined Patent Application Laid-open No. 2007-287771